The goal of this project is to understand the functions of Drosophila RNA binding proteins, particularly those related to the vertebrate A/B group hnRNP proteins, a major class of nuclear proteins that bind pre-mRNA. Biochemical studies of vertebrate hnRNP proteins suggest that they are involved in packaging and splicing nascent transcripts. We use genetics and molecular biology to study six Drosophila proteins that have similar RNA binding domains, two of which, Hrb98DE and Hrb87F, are hnRNP proteins. Four of these proteins are required for male fertility. We isolated a small deletion that removes the Hrb87F gene and part of an adjacent gene, tsr. The only obvious phenotype associated with this deletion is that males are semisterile. The tsr protein has RNA binding motifs, and can bind RNA in vitro. The tsr gene is transcribed only in the male germ line, beginning during the third larval instar. Experiments using fly strains containing transgenic copies of wild type Hrb87F or tsr genes have shown that the disruption of the tsr gene is responsible for the sterile phenotype. In an otherwise wild type background, Hrb87F is not required. However, genetic interaction experiments show that it does affect male fertility under conditions of reduced dosage of Hrb98DE. Mutations in the Rb97D gene, another RNA binding protein, also lead to male sterility. These mutations affect the expression of a dynein protein, an important component of sperm tails. The phenotype of the Rb97D mutants is very similar to that of a deletion of the dynein gene on the Y chromosome, suggesting that we have identified a target of the Rb97D protein. The cabeza gene encodes an RNA binding protein that is a Drosophila homolog of several human proteins that are disrupted in translocations associated with some sarcomas. Several transcripts are produced from the cabeza locus, including one that appears to lack the RNA binding domain. The cabeza protein is predominantly nuclear, enriched in the embryonic brain and nerve cord and in the adult head, and is expressed in the nerve cord and imaginal discs in larvae. The cabeza gene has been mapped to 14B, and a genetic analysis of this locus is underway.